Variable speed drive for subsea installations, vessels, and/or vehicles

ABSTRACT

A Variable Speed Drive ( 1 ) for subsea installations ( 20 ), subsea vessels or subsea vehicles, as well as to a corresponding subsea installation ( 20 ), subsea vessel or subsea vehicle, has an alternating current/alternating current converter ( 2 ) having a current-controlled capacitor-less direct current link ( 7 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a United States national phase filing under 35U.S.C. §371 of International Application No. PCT/EP2006/010614, filedNov. 6, 2006. The complete disclosure of the above-identifiedapplication is hereby fully incorporated herein by reference.

TECHNICAL FIELD

The invention relates to a subsea installation with a Variable SpeedDrive (VSD). Furthermore, it relates to subsea applications such assubsea vessels and subsea vehicles with such subsea installation.

BACKGROUND

Adjustable speed ac induction motor drive is known from U.S. Pat. No.4,137,489 and makes use of a common voltage converter for applying avoltage of variable magnitude to a dc link and a polyphase currentsource inverter having a variable frequency output with the dc linkcurrent magnitude.

As far as the invention is concerned, subsea installations, vessels andvehicles are usually used in exploring and exploiting gas and oil fieldsat wellheads located at the seabed. Variable Speed Drives are used inthis context for applications requiring translatory or rotationalmovements by electric motors.

Prior art Variable Speed Drives for such subsea installations, vesselsand vehicles make use of common voltage-controlled alternatingcurrent/alternating current (AC/AC) converters based on insulated gatebipolar transistors (IGBT), the converters comprising a rectifier, adirect current (DC) link buffered by a capacitor bank, and an inverter.

Disadvantageously, capacitors are sensitive to environment pressure.Prior art subsea applications therefore have to provide pressurecompensation with an internal air pressure equal to the atmosphericstandard pressure of approximately 1013 hPa (1 atm). Typically, thealternating current/alternating current converter and its controlelectronics are placed in a special housing to keep thepressure-compensated volume small. Only the interior of the housing hasto be kept at the atmospheric standard pressure then. As this housinghas to be pressure-resistant against the overpressure of the subseaenvironment or of a surrounding main vessel, several constructionalproblems occur. Securely withstanding the pressure requires thick walls(up to more than 100 mm depending on pressure and dimensions of thehousing) and/or support rods, which takes much space, and poses highprerequisites regarding the sealing of the housing, in particularregarding penetrators for electrical connections into and out of thehousing. Additionally, the converter circuits need a complex coolingsystem to dissipate waste heat from within their housing.

SUMMARY

According to various embodiments, a subsea installation can be specifiedwith a variable speed drive of the type initially mentioned, havinglower constructional requirements. According to further embodiments acorresponding subsea vessel and subsea vehicle can be specified.

According to an embodiment, a subsea installation may comprise aVariable Speed Drive with an alternating current/alternating currentconverter having a current-controlled capacitor-less direct currentlink, wherein at least the converter is arranged within a liquid-tighthousing.

According to a further embodiment, the direct current link may comprisea smoothing inductor. According to a further embodiment, the directcurrent link may consist of the inductor only. According to a furtherembodiment, the housing can be filled with an electrically isolatingliquid. According to a further embodiment, a wall of the housing mayhave a thickness of maximally 10 mm. According to a further embodiment,a volume compensator can be located on the housing. According to afurther embodiment, the housing may be arranged within a liquid-tightmain vessel. According to a further embodiment, the main vessel may beat least partially filled with an electrically isolating liquid.According to a further embodiment, the converter may comprise athyristor-controlled rectifier and a thyristor-controlledline-commutated inverter.

According to another embodiment, a subsea vessel may comprise such asubsea installation with a Variable Speed Drive as described above.

According to yet another embodiment, a subsea vehicle may comprise sucha subsea installation with a Variable Speed Drive as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, exemplary embodiments of the invention are explainedin further detail with drawings.

In the drawings,

FIG. 1 shows a circuit diagram of a variable speed drive;

FIG. 2 shows a circuit diagram of another variable speed drive; and

FIG. 3 shows a subsea installation.

Like parts are denoted by like reference signs in all figures.

DETAILED DESCRIPTION

According to various embodiments, a subsea installation with a variablespeed drive comprises an alternating current/alternating currentconverter having a current-controlled capacitor-less direct currentlink. This not only reduces the number of electronic components in theconverter, hence reducing the probability of failure. Moreover, complexmeasures for pressure compensation can be completely omitted or can atleast be limited to the control electronics of the converter, asaccording to various embodiments no pressure-sensitive capacitors areused. The converter can be exposed to overpressure therefore, inparticular to subsea environmental pressure. The converter can beequipped with any kind of power switches, such as thyristors, metaloxide semiconductor field effect transistors (MOSFET) and insulated gatebipolar transistors.

Preferably, said direct current link comprises a smoothing inductor. Asmoothing inductor is typically used for high-power current-controlleddirect current links. The direct current link may comprise otherpressure-proof components.

Particularly, said direct current link may consist of said inductoronly. This way, the variable speed drive has a simple design and a lowfailure probability and takes only little space.

In a preferred embodiment, at least said converter is arranged within aliquid-tight housing. This housing is preferably filled with a liquid.As liquids are nearly incompressible the liquid serves for supportingthe housing on all sides against any external pressure. As aconsequence, the housing itself is nearly incompressible even at a highexternal pressure such as in the deep sea, and the converter componentsare indirectly exposed to the external pressure. For this reasons, theconstruction complexity of the housing can be significantly reduced. Theseals and fittings, in particular of penetrators, can be simpler and areconsiderably more reliable due to low differential pressure betweeninside and outside of the housing. Additionally, the liquid can serve asa coolant for dissipating waste heat from the converter circuits to theoutside of the housing. The complexity of a cooling system is thereforereduced, too. Because the constructional requirements are lower than inprior art, compact and lightweight Variable Speed Drives are possible bythis embodiment.

Advantageously, said housing is filled with an electrically isolatingliquid. In this case, no precautions have to be taken against shortcircuits between bare electrical contacts of the converter. For example,the liquid can be oil.

Preferably, a wall of said housing has a thickness of maximally 10 mm,in particular of maximally 5 mm. Therefore, the housing has a low heatcapacity, but a high heat conductance for efficiently dissipating wasteheat. Besides, it needs less material, and less space in the subseaapplication. Generally, the walls of said housing only need to have athickness designed to withstand normal handling and operationconditions.

Malfunction due to a leakage can be retarded by a volume compensatorthat is located inside of said housing. The volume compensator preventsexternal liquids from intruding into the converter housing for a certainperiod of time by providing a slight overpressure inside the housing.

In one possible embodiment, said housing is arranged within aliquid-tight main vessel. Other parts of the subsea application can bearranged in the main vessel. In this embodiment, the converter iscontained in the housing independently from such parts and from theinternal pressure of the main vessel. In particular, the main vessel canbe filled with a liquid, hence applying the pressure-resistant principleof the converter housing to the main vessel, too. The main vessel canthus have a reduced construction complexity as the nearly incompressibleliquid serves for supporting the main vessel on all sides. It ispossible to locate the control electronics of the converter in a smallpressure-compensated containment inside or outside of the main vessel.In particular, this pressure-compensated containment can be arrangedinside the converter housing.

Preferably, said main vessel is at least partially filled with anelectrically isolating liquid, for example, oil. The main vessel maycomprise bare electrical contacts in its interior as described above forthe housing of the converter, so short circuits are avoided this way.Besides, in case of a leakage of the housing, the liquid of the mainvessel may enter the housing without electrically influencing theoperation of the converter.

In a special embodiment, said converter comprises a thyristor-controlledrectifier and a thyristor-controlled line-commutated inverter. Insteadof thyristors, it may comprise arbitrary other semiconductor valves.This allows for operating a synchronous machine using the Variable SpeedDrive.

In addition, other embodiments comprise a subsea installation, a subseavessel and a subsea vehicle comprising a Variable Speed Drive asdescribed above.

FIG. 1 shows a sample circuit diagram of a variable speed drive 1 for asubsea installation (not shown in this figure) according to variousembodiments. It comprises a three-phase alternating current/alternatingcurrent converter 2, control electronics 3 and an asynchronous inductionmotor 4. The input of the converter 2 is connected to a three-phasevoltage source 5. The converter 2 supplies the motor 4 with a pulsedthree-phase alternating current.

The converter 2 comprises a rectifier 6, a direct current link 7 and aninverter 8. The exemplary rectifier 6 is a full wave three-phase bridgerectifier comprising six thyristors 9. The exemplary direct current link7 is current-controlled and comprises only a smoothing inductor 10, butno capacitors. The exemplary inverter 8 comprises six thyristors 9,serving for creating pulse-width modulated output waveforms that cause asinusoidal current in the motor 4. The pulse-width modulation isperformed by the control electronics 3. Any algorithm for generating theoutput waveforms can be used by the control electronics 3, for example,space vector modulation.

The converter 2 can be exposed directly or indirectly to a pressuresignificantly higher or lower than 1013 hPa without influencing itsfunction, because no pressure-sensitive capacitors are used.

In another embodiment (not shown), the converter 2 may be contained in aliquid-tight housing that is filled with an electrically isolatingliquid. It supports the housing on all sides against any externalpressure. Hence, the differential pressure between the interior of thehousing and the exterior is negligible. As a consequence, the completedesign of the converter 2 is insensitive to leakages, because theprobability of an external medium intruding into it is strongly limited.Hence, the converter 2 has a high reliability at low production costs.

FIG. 2 shows a sample circuit diagram of another Variable Speed Drive 1according to various embodiments. It is built similar to the one ofFIG. 1. However, the inverter 8 comprises six insulated gate bipolartransistors 11 instead of thyristors, and the motor 4 is a synchronousmachine. Using insulated gate bipolar transistors 11 allows for applyingother modulation patterns for the inverter 8. The ability to switch offthe current at desired times enables the converter 2 to drive otherkinds of motors and loads than synchronous machines, too.

In FIG. 3, an example of a subsea installation 20 is depicted. Itcomprises a liquid-tight main vessel 21 surrounded by seawater 22 andlocated at the seabed 23. The subsea installation 20 is part of awellhead for drilling (not shown) into the seabed 23. A liquid-tighthousing 24 for a converter 2 of a variable speed drive 1 is arrangedinside the main vessel 21. The converter 2 is located inside the housing24. The control electronics 3 of the converter 2 are located inside aseparate air-filled and pressure-compensated containment 25 that islocated, for example, inside the converter housing 24. The housing 24 isfilled with an electrically isolating first liquid 26. The main vessel21 is filled with an electrically isolating second liquid 27. Bothliquids 26, 27 are oils.

The subsea installation 20 is connected to a subsea power line 28providing it with electrical energy in the form of a 10 kV three-phasealternating current. The power line voltage is transformed to a 240 Vthree-phase installation voltage to which the input of the converter 2of the variable speed drive 1 is connected, wherein the transformer canbe seen as a voltage source 5. The converter 2 is designed as the onedescribed in FIG. 1. However, in other embodiments (not shown) it may beany kind of alternating current/alternating current converter comprisinga current-controlled capacitor-less direct current link. The converter 2outputs a 3 kV (root mean square value) three-phase pulse-widthmodulated alternating current to a drilling motor 4.

The voltages given above just describe an exemplary subsea installation20. Other voltages may apply depending on the length of the power line28, the distance from the voltage source 5 to the converter 2, and therequired voltage and power of the motor 4, respectively. If appropriate,the transformer can be omitted.

Both liquids 26, 27 serve as coolants for the respective electroniccomponents. The waste heat rising at the converter 2 inside the housing24 is dissipated via the first liquid 26 to and through the housing 24.From there, it is dissipated via the second liquid 27 to and through themain vessel 21 and, finally, to the seawater 22. For this purpose, acircle flow of the first liquid 26 can be created inside the housing 24,either by the force of gravity or by at least one dedicated pump (notshown). In the same way, a circle flow of the second liquid 27 can becreated inside the main vessel 21. Heat exchanging devices may bearranged in the first liquid 26 and/or in the second liquid 27 toincrease efficiency, wherein one liquid 26, 27 is extensively conductedthrough the other 27, 26 without merging them.

The liquids 26, 27 support the housing 24 and the main vessel 21,respectively, on all sides against the external subsea overpressure. Dueto this, the differential pressures between the interior of the subseainstallation 20, i.e. inside the housing 24 and inside the main vessel21, and the surrounding seawater 22 is negligible. The seals andfittings (not shown), in particular of penetrators, of the housing 24and of the main vessel 21 can therefore be designed for a lowdifferential pressure. In the subsea installation 20 the intrusion ofseawater into the converter 2 even is nearly impossible, because thehousing 24 is located within the main vessel 21. In case of a leakage ofthe main vessel 21 no seawater will enter the housing 24. In case of aleakage of the housing 24 only, the second liquid 27 of the main vessel21 can enter the housing 24. However, it will not lead to malfunction ofthe converter 2, as might be the case for seawater 22 directly intrudinginto the converter 2, because the second liquid 27 is electricallyisolating. Thus, the subsea installation 20 will maintain operation. Dueto these reasons, the Variable Speed Drive 1 is applicable to a widerange of power and voltage.

In other embodiments (not shown), the variable speed drive 1 can be usedfor driving a pump motor 4 or any other kind of motor 4.

FIG. 4 shows a typical application of a Variable Speed Drive 1 in asubsea installation (not shown). The motor 4 and driven equipment arelocated apart from the converter 2 that is arranged in a singleliquid-tight housing 24 filled with a first liquid 26. The controlelectronics 3, if needed, are preferably located in apressure-compensated containment 25 inside of the housing 24. Thehousing 24 is directly exposed to seawater 22. It can be equipped with avolume compensator 29 giving the first liquid 26 a slight overpressurewith respect to the surrounding seawater 22. In case of any leakage thedifferential pressure will prevent the seawater from entering thehousing. The volume compensator 29 can be monitored as necessary.

A volume compensator 29 can also be used in embodiments that comprise amain vessel 21, such as the one described in FIG. 3. The volumecompensator 29 is arranged normally on and alternatively inside thehousing 24 then as described above. Additionally or alternatively, avolume compensator can be arranged inside the main vessel 21, outside ofthe housing 24.

The invention claimed is:
 1. A subsea installation comprising: aVariable Speed Drive; an alternating current/alternating currentconverter; and a liquid-tight housing fully filled with an electricallyisolating liquid, wherein the liquid structurally supports the housingon all sides against external subsea pressure when the housing isarranged underwater; control electronics for the converter locatedinside a gas-filled control electronics containment, wherein at leastsaid converter and said gas-filled control electronics containment arearranged within the liquid-tight housing, with the converter arrangedoutside the control electronics containment, and wherein said gas-filledcontrol electronics containment is internally pressurized to compensatefor external pressure imparted on the containment by the electricallyisolating liquid in the liquid-tight housing.
 2. The subsea installationaccording to claim 1, wherein said direct current link comprises asmoothing inductor.
 3. The subsea installation according to claim 2,wherein said direct current link consists of said inductor only.
 4. Thesubsea installation according to claim 1, wherein a wall of said housinghas a thickness of maximally 10 mm.
 5. The subsea installation accordingto claim 1, wherein a volume compensator is located on or inside saidhousing.
 6. The subsea installation according to claim 1, wherein saidliquid-tight housing is arranged within a liquid-tight main vessel thatis fully filled with a second electrically isolating liquid, wherein theliquid-tight housing is arranged inside the liquid-tight main vesselsuch that the second electrically isolating liquid fills a volumedefined between the liquid-tight main vessel and the liquid-tighthousing.
 7. The subsea installation according to claim 1, wherein saidconverter comprises a thyristor-controlled rectifier and athyristor-controlled line-commutated inverter.
 8. A subsea vesselcomprising: a subsea installation; a Variable Speed Drive; analternating current/alternating current converter; and a liquid-tighthousing fully filled with an electrically isolating liquid, wherein theliquid structurally supports the housing on all sides against externalsubsea pressure when the housing is arranged underwater; controlelectronics for the converter located inside a gas-filled controlelectronic containment, wherein at least said converter and saidgas-filled control electronics containment are arranged within theliquid-tight housing, with the converter arranged outside the controlelectronics containment, and wherein said gas-filled control electronicscontainment is internally pressurized to compensate for externalpressure imparted on the containment by the electrically isolatingliquid in the liquid-tight housing.
 9. A subsea vehicle comprising: asubsea installation with a Variable Speed Drive comprising analternating current/alternating current converter and controlelectronics for the converter located inside a gas-filled controlelectronic containment, wherein at least said converter and saidgas-filled control electronics containment are arranged within aliquid-tight housing that is fully filled with an electrically isolatingliquid that structurally supports the housing on all sides againstexternal subsea pressure when the housing is arranged underwater, andwherein said gas-filled control electronics containment is internallypressurized to compensate for external pressure imparted on thecontainment by the electrically isolating liquid in the liquid-tighthousing.
 10. The subsea vessel according to claim 8, wherein said directcurrent link comprises a smoothing inductor.
 11. The subsea vesselaccording to claim 10, wherein said direct current link consists of saidinductor only.
 12. The subsea vessel according to claim 8, wherein awall of said housing has a thickness of maximally 10 mm.
 13. The subseavessel according to claim 8, wherein a volume compensator is located onor inside said housing.
 14. The subsea vessel according to claim 8,wherein said liquid-tight housing is arranged within a liquid-tight mainvessel that is fully filled with a second electrically isolating liquid,wherein the liquid-tight housing is arranged inside the liquid-tightmain vessel such that the second electrically isolating liquid fills avolume defined between the liquid-tight main vessel and the liquid-tighthousing.
 15. The subsea vessel according to claim 8, wherein saidconverter comprises a thyristor-controlled rectifier and athyristor-controlled line-commutated inverter.
 16. The subseainstallation according to claim 1, wherein a volume compensator islocated inside said housing.
 17. The subsea installation according toclaim 1, wherein the control electronics containment is air-filled. 18.The subsea installation according to claim 1, wherein both theelectrically isolating liquid and the second electrically isolatingliquid are oils.
 19. The subsea installation according to claim 1,wherein the alternating current/alternating current converter has acurrent-controlled capacitor-less direct current link.